Method and apparatus for electrochemical milling



June 30, 1964 P. A. OELGOETZ 3,139,394

METHOD AND APPARATUS FOR ELECTRO-CHEMICAL MILLING Filed June 20, 1962 2Sheets-Sheet 1 INV EN TOR.

PAUL A. OELGOETZ ATTORNEY June 30, 1964 t P. A. OELGOETZ 3,139,394

METHOD AND APPARATUS FOR ELECTRO-CHEMICAL MILLING Filed June 20, 1962 2Sheets-Sheet 2 IN V TUR- PAUL A. OEL ETZ ATTORNEY United States Patent3,139,394 METHOD AND APPARATUS FOR ELECTRO- CHEMICAL MELING Paul A.Gelgoetz, Columbus, Ohio, assignor to North American Aviation, Inc.Filed June 26, 1962, Ser. No. 203,956 12 Claims. (Cl. 204-143) Thisinvention pertains to method and apparatus for metal removal, and moreparticularly concerns method and apparatus for electro-chemicallyremoving metal from regions adjacent selected inset surface areas of asteel panel or the like.

Numerous problems have been encountered in connection with attempts touniformly remove metal from selected surface areas of relatively thin,panel-like, steel aircraft components which are typically contoured toeither a flat, curved, or irregular configuration, or combinationsthereof, prior to the metal removal operation. Generally, metal removalfrom panel surface areas is difficult in that it must be to a uniformdepth with respect to the reference surface of the part, and furthermust be effected without introducing adverse stresses, as bymachine-tool workhardening, into the part. Also, it is important thatmetal removed from panel regions adjacent inset surface areas be carriedout in a manner whereby properly proportioned fillets are provided todefine the edges of the resulting milled surface areas.

Typical acid etching techniques have not proven acceptable for use inmilling aircraft panel components fabricated of the highly-alloyed orheat-treatable steels because such materials are especially susceptibleto hydrogen embrittlement. Similar steel parts subjected cathodically toan electrolytic acid etching treatment will also develop hydrogenembrittlement.

Conventional electro-chemical metal removal methods and apparatus havefurther proven deficient in that steel parts milled thereby have notbeen etched at uniform rates of metal removal throughout surface areasof substantial extent. Such unevenness in depth of metal removal isgenerally not acceptable when related to the production of aircraftcomponents having stringent weight limitations and maximum stressconditions associated therewith.

Problems have also been encountered with respect to the intergranularcorrosion introduced into certain steel workpieces when milled throughuse of known acid etching techniques. In addition, the practice ofelectro-chemical milling metal removal techniques using state-of-theartapparatus has posed problems with respect to obtain ing consistenttolerances of small magnitude substantially throughout surface areas ofconsiderable extent. Further, the use of known methods and apparatus forthe stated purpose of electro-chemically removing steel from surfaceareas of steel panels has required the use of more electrical energythan is comparatively required in connection with the practice of thisinvention. Apparatus heretofore developed for elcctro-chemicallyremoving metal from regions adjacent selected inset surface areas of asteel-like panel have often employed a bleeder means member which is notrequired by this invention to obtain proper milled area edge filletcharacteristics.

Accordingly, it is an object of this invention to provide apparatus formetal removal which will result in acceptably smooth final surfacefinishes and which further will not introduce Work-hardening effectsinto steel parts subjected thereto.

Another object of this invention is to provide an elec-i tro-chemicalmilling equipment arrangement that does not expose worked-upon highlyalloyed steel parts to the phenomenon of hydrogen embrittlement.

Another object of this invention is to provide electrochemical millingtooling which is effective to remove metal from extended or substantialsurface areas of steel materials at uniform metal-removal rates.

Another object of this invention is to provide apparatus which may beused for uniformly removing metal from pre-formed steel parts which haveeither flat, curved, or irregular surface configurations.

Another object of this invention is to provide apparatus which may beadvantageously utilized to electro-chemically remove metal from regionsadjacent selected inset surface areas of a steel panel or the like to auniform depth substantially over the entire resulting milled area.

Another object of this invention is to provide improved apparatus forelectro-chemically removing metal from selected inset regions of asteel-like panel in a manner which develops a properly proportionedfillet at the edge which defines the resulting milled surface area.

A still further object of my invention is to provide ap paratus andequipment for removing metal from ultra high-strength steel panels whichdoes not subject such 7 panels to the phenomenon of intergranularcorrosion.

Another object of this invention is to provide apparatus forelectro-chemically removing metal from selected regions of a steel-likepanel with a reduced requirement as to total electrical energy.

Another object of this invention is to provide apparatus forelectro-chemically removing metal from steel workpieces in a mannerwhich obtains improved tolerances of greater consistency throughoutsurface areas of considerable size.

Another object of this invention is to provide apparatus which utilizescommercially-available materials and equipment for operation andconstruction. Additionally, the equipment utilized for the process ofthis invention is comparatively simple in construction, is operable bysemiskilled operating personnel, and is free of complex main tenancedifiiculties.

Other objects and advantages of this invention will become more apparentwhen consideration is given to the drawings and written descriptionforming portions of this application.

In the drawings:

FIG. 1 illustrates component parts of the apparatus of this invention inassembled relation;

FIG. 2 illustrates component parts of the apparatus of FIG. 1 inexploded relation;

FIG. 3 illustrates masking details of the workpiece and cathodecomponents of the apparatus of this invention;

FIG. 4 illustrates a detail shown in FIG. 3;

FIG. 5 is a sectional view which further illustrates the relation ofopposed unmasked surface areas of the workpiece and cathode componentsof my invention;

FIG. 6 is an enlarged partial elevational view of the apparatusillustrated in FIG. 1;

FIG. 7 is a sectional view taken at line 7-7 of FIG. 6;

FIG. 8 is a sectional view taken at line 8-8 of FIG. 6;

FIG. 9 is a sectional view taken at line 9-9 of FIG. 6;

FIG. 10 illustrates the preferred workpiece milled surface area edgefillet condition Which is obtained through practice of this invention;and I FIGS. 11 and 12 illustrate other workpiece milled surface areaedge fillet conditions;

FIG. 13 illustrates an equipment arrangement employed with the apparatuscomponents detailed in FIGS. 1 through 9.

FIG. 1 illustrates component parts of the apparatus of this invention inassembled relation. The assembly is referenced generally as 10 and isbasically comprised of an anode member 11, a cathode member 12, and theworkpiece designated 13. Each of the components is provided in selectedareas with an adhering dielectric filrn that is chemically-resistant toelectrolyte and that material such as methyl methacrylate.

is preferably manually strippable from underlying com- I maskant.Supportmembers 14 and 15 are mechanically coupled and electricallyconnected to anode member 11 and cathode member 12, respectively.

Details with respect to components 11, 12, and 13 and with respect toother components of assembly 10 are shown in exploded relation in FIG.2. Anode member 11 is fabricated of metal and is coated with the maskantdesignated 16 (FIG. 7). Openings 17 are provided in anode member 11 forreceiving insulating bushings 18 and the threaded screws designated 19that mechanically couple anode member 11 to cathode member 12 but inelectrically-insulated relation. Threaded screws 19 pass through thespacers designated 21 and through openings 21 in cathode member 12 andcooperate with the nuts designated 22. See FIG. 7. Bushings 18 andspacers are preferably fabricated of a comparatively rigidchemically-resistant and electrically-resistant Openings 23 in anodemember 11 are provided for receiving screws members 24 that electricallycouple anode member 11 to workpiece 13. A portion of the maskant 16provided on the rearmost surface of anode member 11 is removed fromregions immediately adjacent each opening 23. This is accomplished inorder that the nut 25 which cooperates with screw member 24 might makean electrically-conducting contact between nut member 25 and anodemember 11. A portion of the maskant 16 provided on the rearmost surfaceof anode member 11 in the vicinity of the region designated 25 is alsoremoved in order to establish an electrically-conducting contact betweenanode member 11 and support member 1 The fastener designated 27 isprovided to removably couple support member 14 to anode member 11 at theregion designated 26. Further details of this feature are provided inconnection with the discussion of FIG. 9.

Cathode member 12 is also fabricated of a metal and is provided with theprotective coating designated 28. Cathode member 12 is provided withopenings 29 for receiving the insulating bushings designated 30. Eachbushing 31) is provided to assure that a screw member 24 is notelectrically-connected to cathode member 12 during its location within ahole 29 of cathode member 12. Open inset areas 31 are provided incoating 28 in prescribed locations to assist in obtaining the objectivesof this invention. Cathode member 12 is also provided with a number ofinsulating bushings 32 and locating pins 33. Bushings 32 are preferablypress-fit into the openings designated 34. Locating pins 33 may beeither fixedly or removably located in bushings 32. Openings 35 areprovided in cathode member 12 to facilitate immersion and removal ofapparatus 10 in an electrolyte bath by use of a conventional hoist andhook. Support member 15 is connected to region as of cathode mem ber 12by fastener means 37 and in the manner of the attachment of supportmember 14 to anode member 11.

Workpiece 13 is provided with the protective film designated 38.Workpiece 13 contains the openings designated 39 for receiving locatingpins 33 to establish the preferred proper positioning of workpiece 13relative to cathode member 12. Openings 40 are provided in workpiece 13for receiving screw members 24. Each screw member 24 projects through anopening 40 and cooperates with a nut designated 41. Because nut 41constitutes a part of the circuit which electrically connects workpiece13 to anode member 11, a portion of the mask 38 provided on workpiece 13must be removed from adjacent each opening 40 to'permit metal-to-metalcontact between nut member 41 and workpiece 13. Spacer members 42 serveto maintain workpiece 13 a proper distance from cathode member 12. Inthe arrangement shown in FIG. 2, screw members 24 pass through spacermembers 42. Seals 43 and cap members ll. 44 are provided to developchemical and electrical protection for the exposed end of screw member24 projected beyond workpiece 13 and for nut 41.

Illustrations showing details of the manner in which anode member 11 andcathode member 12 are mechanically coupled to each other inelectrically-insulated relation, of the manner in which workpiece 13 islocated with respect to cathode l2, and of the manner for electricallyconnecting workpiece 13 to anode member 11 are shown in FIGS. 6 through8. FIG. 9 discloses details of a preferred manner for securing'supportmember 14 to anode member 11 in a manner which prevents rotation of suchcomponents relative to each other. The FIG. 9 arrangement employsindividual elongated recesses 50 in region 26 of anode member 11 and onesimilar recess 51 in support member 14. A small key 52 cooperates withrecess 51 and with a recess 50 in anode member 11 to prevent rotation ofthe components when fastener 27 is properly installed. Such arrangementis employed to facilitate repositioning of the support members to permitupside-down immersion of the tooling assembly in the electrolytepart-way through the metal removal operation.

Important advantages of my invention are achieved through the propermasking or coating of workpiece 13 and cathode member 12. To accomplishsuch advantages it is necesary to properly select, locate, size, andposition various openings in the coatings 28 and 38 of components 12 and13. Such openings are designated 31 and 45 (FIGS. 3 through 5) and aredeveloped in the related component part by proper removal of selectedportions. In applying the coatings to components 12 and 13 (and alsocoating 16 to anode member 11) it is preferred that such componentsfirst be cleaned using conventional degreasing equipment or the like. Itis also preferred tht the component parts be subjected to moderate sandblasting in order that uniform adhesion of the subsequently-appliedcoating might be obtained. A precoating surface roughness ofapproximately 100 microinches R.M.S. generally proves entirelysatisfactory for this purpose. Coatings 28 and 38 are preferably nextapplied over the entire exterior surface of each component through useof either a standard spraying or dipping technique. Commerciallyavailable materials suitable for the coating included modified vinylpolymers, cellulose acetate butyrate, polyethylene, or the like. Oneparticularly effective air-drying maskant material is comprised ofapproximatley 18% polyvinyl chloride, 2% cellulose acetate butyratemodifier, and approximately 80% solvent such as methyl isobutyl ketoneand methyl ethyl ketone, all percentages being on a weight basis.Adequate chemical and electrical resistance is generally obtained byapplying three (3) wet cross-coats of the composition to the surfaces ofthe tooling assembly component parts. Suificient time should be allowedbetween the application of each maskant coat to permit the solventscontained therein to sufficiently evaporate. The

. final coating is readily completely manually strippable and has athickness generally approximating .003". Other comparable commercialcoatings, such as those requiring a high-temperature baking step asdistinguished from air-drying, may be used in the practice of thisinvention.

Particular details with respect to properly defining and locatingspecific coating openings in tooling assembly component parts 12 and 13are shown in FIGS. 4 and 5; the details of FIG. 4 are especiallydirected to the areas designated 31a and 45a in FIG. 3. The details andthe following comments also pertain to the other of the coating openingsprovided in the component parts. For the purpose of this discussion itis assumed that metal is to be removed from contoured panel-likeworkpiece 13 to a uniform depth throughout the extent of the areadefined by the line or edge designated 46/ The extent of the depth ofplanned metal removal is shown by the dotted line 47 of FIG. 5. Itshould be noted that a fillet of nearly-constant radius equal to thedepth of metal removal is desirably provided at the workpiece milledsurface area edge region. In practicing this invention, inset portionsof the maskant material separately provided on workpiece 13 are removedfrom area 45a tothe edge designated 49. Edge 49 is inset uniformly fromedge 46 to a distance C. Distance C typically is from /2 to 1 times thedepth of metal removal. Maskant removal can be accomplished by using ahot, sharp stylus and a properly configured and contoured template tocut coating 38 coincident with edge 49. After the edge is properlyscribed in the coating, the inset coating portion may be removed fromthe component part readily by a manual stripping operation. FIG. 4discloses that edge 49 includes parallel straight line portions,diverging straight line portions, and connection radii. One such radiusdefined by edge 46 has a preselected value designated R.

Each coating opening provided in cathode member 12 for cooperation witha corresponding coating opening in workpiece 13 is'of lesser extent inarea than the area defined by edge 49 by a prescribed amount. Referringto the opening 31a in cathode member 12, the defining edge 48 is shapedto be similar to edge 49 and is concentrically located with respect toedge 49. The resulting unprotected surface area 31a of cathode member 12has a similar contour to the contour of the unprotected surface area ofcorresponding opening 45a in FIG. 4. However, each portion ofcorresponding edge 43 of coating 23 is set inwardly the distance B fromthe related edge 49 of the opening 45a. Workpiece 13 and cathode member12 are uniformly spaced-apart the distance designated A in FIG. 5.Distance B is in the range of approximately A to /2 of distance A withthe lower limit being preferred. The offset relation between edges 49and 48 is shown in FIG. 4. In the corner radii regions the cathodemember inset surface area radius is of a dimension B less than theradius dimension for the corresponding part of inset surface area 45a.This olfset relation is required to obtain the heretofore-mentioneddesirable fillet characteristic at the workpiece milled area edgeregion.

The resulting preferred fillet condition at the edge region of theworkpiece milled area is best shown in FIG. 10. It should be noted thatthe disclosed fillet has a nearly-uniformly radius corresponding to thedepth of metal removal. The resulting condition is highly desired insforas the design and manufacture of aircraftquality parts are concerned. Ifthe teachings of this invention are not adhered to, undesirable edgefillet conditions such as those illustrated in FIGS. 11 and 12 mayresult. In the FIG. 11 illustration, the edge 53 provided in maskant 54for the cathode member designated 55 is positioned inwardly asubstantial distance more than the preferred 4 to A2 offset relationwith respect to the edge 49 of coating 38 on the workpiece designated56. The resulting fillet condition does not have the desirednearlyuniform fillet radius that corresponds to the depth of metalremoval; the edge fillet condition of workpiece 56 has an undesirablypronounced taper not present in the fillet condition of workpiece 13.FIG. 12 also discloses an undesirable workpiece milled area edge filletcondition. In that figure, the edge 57 scribed in coating 58 on cathodemember 59 is positioned directly opposite the edge 49 provided inmaskant 38 to workpiece 60. The resulting milled area edge filletundercuts the depth of metal removal. This condition is unwanted insofaras aircraft-quality panels are concerned.

FIG. 13 discloses other component parts used in connection with theapparatus of this invention. Such other component parts include. aprocess tank 61, electrolyte acid solution 62, and the anode and cathodebus bars respectively designated as 63 and 64. Such bus bars areelectrically connected to a source of electrical energy (not shown) andare each provided with a clamp member 65 for effecting an electricalconnection between the bus bar and a support member 14 or 15 of toolingassembly 10 during operation of the apparatus. The source of electricalenergy cooperating with bus bars 63 and 64 provides direct-type currentand may take the form of a source of rectified alternating currentelectrical energy.

Process tank 61 is preferably constructed of, or lined with, a materialthat is chemically-resistant to the acid solution 62 contained therein.Material such as wood, polyvinyl chloride, natural or synthetic rubber,organic materials such as polytetrafluoroethylene, and the like haveproved acceptable and are well-known to those familiar with the art.Acid solution 62 is electrically conducting and typically containsdisassociated hydrogen ions. A solution comprised of 25% 15% sulfuricacid by weight and water is preferred in the practice of this invention.However, it is recognized that other single acids, such as phosphoricacid, and combination of acids will prove adequate when substituted forsulfuric acid. It is believed that minimum maintenance and control isrequired when single acid solutions, such as the above-referenced 25%i5% sulfuric acid solution, are employed. In practicing this inventionit is desirable that the operating temperature of solution 62 bemaintained at approximately F. :10 F., and that the solution becontinuously circulated as by air agitation.

' During the processing of the workpiece in the equipment arrangementcathode member 12 and workpiece 13 are subjected to the electricalenergy potential across bus members 63 and 64. The tooling assembly(10), except for projecting portions of support members 14 and 15, isimmersed in acid solution 62. Oxygen is given off at the face betweenuncoated anodic surface areas 45 and electrolyte solution 62. Hydrogengas is evolved at the uncoated surface areas 31 of cathode member 12.The process must be continued a sufiicient time to remove metal to thedesired depth such as to the dotted line position 47 indicated in FIG.5. To completely avoid hydrogen embrittlement, electrical current shouldbe flowed to and from tooling assembly 10 as it is being both immersedin and removed from the solution bath 62 in process tank 61.

The following information is provided to describe specific apparatusthat has been employed to selectively remove metal from fiat, uncoatedpanel surface areas having maximum dimensions of approximately 4" x 6".The workpiece test panel from which the metal was removed was comprisedof PH 15-7 Mo alloy steel 0.060 thick. Metal was removed to a nominaldepth of 0.052" leaving a remaining metal thickness of 0.008". Thespecific apparatus has also been used to selectively remove metal fromworkpiece panels fabricated of Type H-ll alloy steel and of metals suchas Inconel X, Ren 41, H-25, and N-lSS with equally good results. Theanode member was fabricated of Type 615 aluminum alloy approximately /2"thick. The cathode member employed in the specific apparatus was alsofabricated of Type 615 aluminum alloy approximately /2" thick. Thevarious bushing and spacer devices employed in such apparatus werefabricated of methyl methacrylate and the coatings provided over themajor exterior surface areas of the anode member and of the cathodemember corresponded to the above-described air-drying polyvinyl chloridemaskant material. Screw members employed to connect the cathode memberto the anode member and to connect the workpiece to the anode memberWere made of an aluminum alloy. Locating pins employed in the apparatuswere manufactured of titanium.

Openings provided in the coating on the cathode member for cooperationwith coating openings in the workpiece were developed following thepreviously-described principles. The workpiece was spaced-apart from theopposing similarly contoured surface of the cathode member a uniformdistance of approximately M5" and the inward offset of the edge 48 tothe cathode member coating opening was approximately The same relationwas provided for all coating opening corner radii.

The current density maintained over the uncoated surface areas of theworkpiece was approximately 8 amperes per square inch of to-be-milledsurface. Other current densities in the range of from to 10 amperes persquare inch may be successfully employed. The direct current voltageestablished as between the workpiece and the cathode member varies,depending on size and thickness of the workpiece panel; in the case ofthe herein-described specific apparatus the voltage was established atapproximately 4 volts. Other voltages in the range of from 4 volts to 6volts have been used for the purpose of carrying out this invention. Theacid solution employed in the specific apparatus consisted ,of 25%sulfuric acid by weight and water. The milled surface of the panelworkpiece was examined subsequent to immersion in the acid solution atthe prescribed current density and voltage for approximately 3 hours.Rate of metal removal varies with workpiece composition. The dimensionaltolerance of the milled area was to within 0.001 as to uniformity ofmetal removal. Surface roughness of the milled area was 30 to 40microinches R.M.S. or better. Neither hydrogen embrittlement orintergranular corrosion characteristics were present in the finishedproduct. In addition, the workpiece milled area was provided with anedge fillet condition of desirable characteristic. More specifically,the milled area edge fillet condition was of nearly-uniform radiuscorresponding to the depth of metal removal and without metalundercutting.

It is to be understood that the form of the invention herewith shown anddescribed is to be taken as the preferred embodiment of the same;various changes in shape, size and arrangement of component parts, asfor removing metal from workpiece panels of different configuration, maybe effected without departing from the spirit of the invention or fromthe scope of the claims.

I claim:

1. Apparatus to'be connected to an electrical energy supply forelectro-chemically removing metal to a uniform depth from andsubstantially throughout an anodic workpiece surface area which isdefined by a prescribed edge and which is immersed in an acidelectrolyte solution, and comprising:

(a) A metal cathode member having a surface area which is spaced-apartfrom said workpiece surface area a uniform distance and which iscontoured similarly to said workpiece surface area,

(b) A dielectric coating which is chemically-resistant to saidelectrolyte solution andwhich is adhered over the exterior of saidworkpiece but with an inset opening that is defined by an edge setinwardly from said workpiece surface area prescribed edge a uniformdistance, and

(c) A dielectric coating which is chemically-resistant to saidelectrolyte solution and which is adhered over the exterior of saidcathode member but with an inset opening that is located at said cathodemember surface area and that is defined by an edge,

said cathode member coating inset opening being concentricallypositioned with respect to said workpiece coating inset opening andhaving said cathode member coating inset opening edge set inwardly withrespect to said workpiece coating inset opening edge.

2. The invention defined by claim 1, wherein said cathode member surfacearea is spaced-apart from said workpiece surface area a uniformpreselected distance, said cathode member coating inset opening edgebeing set inwardly from said workpiece coating inset opening edge auniform distance which is approximately to /2 of said preselecteddistance.

3. The invention defined by claim 1, wherein said cathode member surfacearea is spaced-apart from said workpiece surface area a uniformpreselected distance, said cathode member inset opening edge being setinwardly from said workpiece coating inset opening edge auniformdistance which is approximately A of said preselected distance.

4. Apparatus for use with a inetal removal electrolyte and with a sourceof electrical energy to remove metal to a uniform depth fromand'throughout an anode workpiece precontoured surface area which isdefined by a prescribed protective coating edge, and comprising 1 metalcathode means having a surface area contoured to match said workpieceprecontoured surface area and having an adhering protective dielectriccoating which is provided at said cathode means surface area and whichis provided with an inset opening defined by an edge that conforms inconfiguration to and is of lesser extent than'said workpiece protectivecoating edge, said cathode means surface area being spaced apart fromsaid workpiece surface area a uniform distance with said cathode meansprotective coating edge located concentrically with respect to saidworkpiece protective coating edge.

5. The invention defined by claim'4, wherein said cathode means surfacearea is spaced apart from said workpiece surface area a preselecteddistance in a direction transverse to said cathode means surface area,said preselected distance being approximately 2 to 4 times the inwardoffset distance of said cathode means protective coating edge relativeto said workpiece protective coating edge.

6. The invention defined by claim 4, wherein said cathode means surfacearea is spaced apart from said workpiece surface area preselecteddistance in a direction transverse to said cathode means surface area,said preselected distance being approximately 4 times the inward offsetdistance of said cathode means protective coating edge relative to saidworkpiece protective coating edge.

7. In a method of removing metal from an anodic workpiece and to apredetermined depth uniformly throughout an uncoated surface area whichis defined by an acid-resistant dielectric coating edge and which isimmersed in an acid electrolyte with a cooperating cathode member, thesteps of:

(a) Covering a cathode member which has a surface area contoured tocorrespond to the workpiece uncoated surface area with an adheringacid-resistant dielectric coating,

(b) Providing said cathode member coating with a scribed line having aconfiguration which corresponds to the workpiece coating edge but whichis uniformly set inward a first predetermined distance in a directionparallel to the workpiece uncoated surface area,

(c) Removing said cathode member coating from said cathode member withinsaid cathode member coating scribed line, and

(d) Afterwards locating the cathode member in fixed relation withrespect to the workpiece whereby the uncoated surface area defined bysaid cathode member scribed line is concentric with the workpieceuncoated surface area but is uniformly spaced-apart from the workpieceuncoated surface area a second predetermined distance in a directionnormal thereto.

8. The method defined by claim 7, wherein said second predetermineddistance is from 2 to 4 times said first predetermined distance.

,9. The method defined by claim 7, wherein said second predetermineddistance is 4 times said first predetermined distance.

10. In a method of removing metal from an anodic workpiece and to apredetermined depth uniformly throughout an uncoated surface area whichis defined by an acid-resistant dielectric'coating edge and which isimmersed in an acid electrolyte with a cooperating cathode member, thesteps of:

' (a) Providing a cathode member having a surface contoured tocorrespond to the workpiece uncoated surface area with an uncoatedsurface area portion which is defined by an acid-resistant dielectriccoating edge that corresponds in configuration to the workpiece coatingedge but that is uniformly set inward from the workpiece coating edge ina direction parallel to the workpiece uncoated surface area, and

(b) Locating said cathode member in fixed relation with respect to theworkpiece whereby said cathode member uncoated surface area isconcentric with the workpiece uncoated surface area but is uniformlyspaced-apart from the workpiece uncoated surface area a secondpredetermined distance in a direction normal thereto.

11. The method defined by claim 10, wherein said sec- 0nd predetermineddistance is from 2 to 4 times said first predetermined distance.

12. The method defined by claim 10, wherein said second predetermineddistance is 4 times said first predetermined distance.

References Cited in the file of this patent UNITED STATES PATENTS PrestDec. 22, 1942 Greenspan Apr. 13, 1954 Osman June 19, 1956 BommerscheirnDec. 26, 1961 Oelgoetz Ian. 16, 1962 Oelgoetz June 19, 1962 WilliamsJune 26, 1962

7. IN A METHOD OF REMOVING METAL FROM AN ANODIC WORKPIECE AND TO APREDETERMINED DEPTH UNIFORMLY THROUGHOUT AN UNCOATED SURFACE AREA WHICHIS DEFINED BY AN ACID-RESISTANT DIELECTRIC COATING EDGE AND WHICH ISIMMERSED IN AN ACID ELECTROLYTE WITH A COOPERATING CATHODE MEMBER, THESTEPS OF: (A) COVERING A CATHODE MEMBER WHICH HAS A SURFACE AREACONTOURED TO CORRESPOND TO THE WORKPIECE UNCOATED SURFACE AREA WITH ANADHERING ACID-RESISTANT DIELECTRIC COATING, (B) PROVIDING SAID CATHODEMEMBER COATING WITH A SCRIBED LINE HAVING A CONFIRGURATION WHICHCORRESPONDS TO THE WORKPIECE COATING EDGE BUT WHICH IS UNIFORMLY SETINWARD A FIRST PREDETERMINED DISTANCE IN A DIRECTION PARALLEL TO THEWORKPIECE UNCOATED SURFACE AREA, (C) REMOVING SAID CATHODE MEMBERCOATING FROM SAID CATHODE MEMBER WITHIN SAID CATHODE MEMBER COATINGSCRIBED LINE, AND (D) AFTERWARDS LOCATING THE CATHODE MEMBER IN FIXEDRELATION WITH RESPECT TO THE WORKPIECE WHEREBY THE UNCOATED SURFACE AREADEFINED BY SAID CATHODE MEMBER SCRIBED LINE IS CONCENTRIC WITH THEWORKPIECE UNCOATED SURFACE AREA BUT IS UNIFORMLY SPACED-APART FROM THEWORKPIECE UNCOATED SURFACE AREA A SECOND PREDETERMINED DISTANCE IN ADIRECTION NORMAL THERETO.